1. Field of the Invention
The present invention relates to a ball screwed nut defined with a spiral ball rolling groove in which balls roll, a linearly guiding apparatus using the ball screwed nut, a bail screw for steering, and a method of manufacturing the bail screwed nut.
2. Description of the Related Art
Conventionally, a ball screwed nut was defined in an internal groove with a ball rolling groove by cutting and grinding. As a method of defining the ball rolling groove in the ball screwed nut, the following method has been known. At first, a prepared hole is opened in a blank material by a drill. Next, a spiral ball rolling groove is cut in the peripheral surface of the prepared hole by a bit. Finally, the ball rolling groove thus cut is ground on its surface with a grinding stone.
However, in the conventional ball screwed nut defined with the ball rolling groove by the cutting and grinding processes, there is a case where when grinding a ball screwed nut small in inner diameter, a grinding stone cannot be inserted into a prepared hole, thereby making the grinding process impossible. Otherwise, similarly to the ball screwed nut having the small inner diameter, even in a ball screwed nut having a large lead, there is a case in which since an inserting amount of the grinding stone into the prepared hole is limited because a lead angle is large, the grinding process cannot be performed.
Besides, in the grinding process, concave and convex in cross section of a ball rolling groove to be machined are sharpened. As a result, when the balls roll on the processed part, a convex is leveled and worn, the generation of worn powders and the reduction of pre-load by an amount of wearing the convex is induced. Since the grinding stone is fixed at a front end of a shaft which is cantilevered, the grinding stone may be subjected to chattering vibration during grinding operation or probably pushed off from a ground face by bending the shaft of the grinding stone. Consequently, there was a fear that the roughness of surface is more furthered in the conventional art.
Besides, in the grinding process of the ball rolling groove, the grinding stone is liable to be worn because of the small diameter of the grinding stone to lower the precision of shape of the radius of curvature of the ball rolling groove. As a result, the precision of the radius of curvature of the ball rolling groove is liable to be varied, resulting in a variation of the diameter, irregularity in sizing or lowering of degree in roundness.
Accordingly, an object of the invention which has been made in view of the above is to provide a ball screwed nut which can be formed with a ball screw having a ball rolling groove small in diameter or large in lead, and which is small in surface roughness of the ball rolling groove and high in precision of R shape of the ball rolling groove, and a method of manufacturing the ball screwed nut.
To achieve the above object, according to a first aspect of the invention, there is provided a ball screwed nut which is defined in an inner peripheral surface thereof with a ball rolling groove in which balls roll, in which the ball rolling groove is formed by a rolling process.
According to this invention, since the ball rolling is subjected to a rolling process, the ball rolling groove can be formed without the grinding process. Therefore, a ball screwed nut even having a small diameter or a large lead can be produced. Further, by a plastic working employing the rolling process, the surface of the ball rolling groove is processed in a sharp and flat face without creating concave and convex thereon, and is therefore mirror-finished with less roughness. Thus, wearing of the surface of the ball rolling groove caused by rolling the balls is reduced, and the pre-load is maintained, with the result that the durability of the ball screw can be enhanced. In addition, since the ball rolling groove is formed by the rolling process not by the grinding stone which is easily worn but by the plastic working tool such as a rolling tap excellent in the durability, the precision of R shape in the surface of the ball rolling groove can be made excellent.
According to a second aspect of the invention, there is provided the ball screwed nut defined in the first aspect of the invention, in which the ball rolling groove is finished on the surface thereof by a burnishing process.
According to the invention, concave and convex on the surface of the ball rolling groove are evened by the burnishing process into a more smoothened surface.
According to a third aspect of the invention, there is provided the ball screwed nut defined in the first or second aspect of the invention, in which the ball screwed nut is made of a stainless steel corresponding to SUS-27 of Japanese Industrial Standards (JIS), and hardness of the surface of the ball rolling groove which has been subjected to work hardening by the rolling process is 40 or more in C scale of Rockwell hardness.
According to the invention, since metal structure of the ball rolling groove is continued in fibrous texture, the surface of the ball rolling groove is work-hardened to enhance hardness. Since the ball screwed nut is made of a stainless steel corresponding to SUS-27 of Japanese Industrial Standards (JIS), and the hardness of the surface of the ball rolling groove which has been subjected to work hardening is set to 40 or more in C scale of Rockwell hardness, the ball rolling groove having the hardness sufficient to roll the balls can be available.
According to a fourth aspect of the invention, there is provided a linearly guiding apparatus comprising: a ball screwed shaft having a first spiral ball rolling groove in an outer peripheral surface and, a slider supported movably in a longitudinal direction of guide rails and including an endless circuit with a second spiral ball rolling groove corresponding to the first spiral ball rolling groove, and a plurality of balls arranged in line in the endless circuit so as to roll therein in association with movement of the slider, the slider being reciprocated while the ball screwed shaft rotates, wherein the second ball rolling groove of the slider is formed by the rolling process.
According to the invention, in addition to the actions and effects obtained by the first aspect of the invention, even if the slider extends along the longitudinal direction of the guide rail, the ball rolling groove can be formed in the center of the slider. Therefore, a force of reciprocating the slider can be exerted on the center of the slider so as to check twisting of the slider, so that the reciprocation of the slider is well balanced.
According to a fifth aspect of the invention, there is provided the ball screw for steering, comprising: a ball screwed shaft having a first spiral ball rolling groove in an external peripheral surface thereof and connected to a steering wheel; a ball screwed nut having an endless circuit with a second spiral ball rolling groove corresponding to the first ball rolling groove and a steering gear on an external periphery thereof, and a plurality of balls arranged in the endless circuit of the ball screwed nut to roll while receiving a load between the first and second ball rolling grooves, wherein the second ball rolling groove of the ball screwed nut is formed by a balling process.
According to the invention, as described in the first aspect of the invention, since the second ball rolling groove of the ball screwed nut is formed by the rolling process, the surface of the second ball rolling groove is processed in a sharp and flat surface without creating concave and convex thereon, and is mirror-finished with less roughness. Thus, wearing of the surface of the second ball rolling groove caused by rolling the balls is reduced, and a pre-load is maintained, so that the durability of the ball screw can be improved. In addition, the ball rolling groove is formed by the rolling process not by a grinding stone which is readily worn but by a plastic working tool such as a rolling tap excellent in the durability, so that the precision of R shape in the surface of the ball rolling groove can be made excellent.
According to a sixth aspect of the invention, there is provided a method of manufacturing a ball screwed nut defined with a spiral ball rolling groove in which balls roll, said method comprising the steps of: forming a prepared hole in a blank material, and rolling the ball rolling groove by relatively rotating the blank material and a plastic working tool while inserting the plastic working tool into the prepared hole. A rolling tap may be used to the plastic working tool.
According to the invention, since there is provided the step of rolling the ball rolling groove in the ball screwed nut, one process of the rolling process is sufficient, although the conventional method requires two processes consisting of the cutting and grinding processes, whereby the processing cost for the ball rolling groove can be lowered, and a lead time can be shortened as the processing steps are saved.
According to a seventh aspect of the invention, there is provided the method as defined in the sixth aspect of the invention, further comprising the step of cutting the spiral groove in the inner peripheral surface of the prepared hole, wherein the rolling step includes the sub-step of screwing the plastic working tool in the spiral groove.
According to the invention, since there is provided the step of cutting the spiral groove in the prepared hole, a processing resistance of the plastic working tool is reduced in the rolling process, and the ball screwed nut having the ball rolling groove large in diameter processing resistance can be formed by the rolling process.
According to an eighth aspect of the invention, there is provided the method as defined in the seventh aspect of the invention, wherein a groove to be formed by the cutting process is almost V-shaped in cross section.
According to the invention, since the groove of substantially V-shaped in cross section is cut in the cutting process, when it is plastic-worked by the plastic working tool, a flow of the blank material caused by pressure of the plastic working tool is diverged into two directions, that is, in a direction of a bottom having the R shape in cross section and outwardly thereof at a first contacting point between the plastic working tool and the V shaped groove as the flow dividing point. Therefore, compared with a case in which a flat prepared hole not formed with the V shaped groove is plastic-worked and the blank material is caused to flow from the R-shaped bottom outwardly, a flowing distance of the blank material can be shortened, so that a microscopic spring back caused by the plastic deformation of the ball rolling groove can be decreased and the precision of the ball rolling groove can be enhanced.
According to a ninth aspect of the invention, there is provided the method as defined in any one of the sixth to eighth aspects of the invention, wherein the surface of the ball rolling groove is finished by the burnishing process.
According to the invention, since the plastic working tool having a smooth surface is pressed to the surface of the ball rolling groove, the surface of the ball rolling groove comes to a more smoothened mirror face by the burnishing work.